MOSCOW (AP) — Fifteen people died and scores were wounded Sunday as a female suicide bomber struck at a railway station in southern Russia, officials said, heightening concern about terrorism ahead of February’s Olympics in the Black Sea resort of Sochi.

No one immediately claimed responsibility for Sunday’s attack in Volgograd, but it came several months after Chechen rebel leader Doku Umarov called for new attacks against civilian targets in Russia, including the Sochi Games.

Suicide bombings and other attacks linked to Islamic rebels roaming the North Caucasus have rocked Russia for years. The government has deployed tens of thousands of soldiers, police and other security personnel to protect the Olympics, President Vladimir Putin’s pet project, and the organizers have pledged to make the Sochi Games the “safest Olympics in history.”

Vladimir Markin, the spokesman for the nation’s top investigative agency, the Investigative Committee, said the suicide bomber detonated her explosives in front of a metal detector just behind the station’s main entrance.

“When the suicide bomber saw a policeman near a metal detector, she became nervous and set off her explosive device,” Markin said in a statement. He added that the bomb contained about 10 kilograms (22 pounds) of TNT and was rigged with shrapnel.

Markin said that security controls prevented a far greater number of casualties at the station, which was packed with people at a time when several trains were delayed.

Markin said that 13 people and the bomber were killed on the spot and another victim died later at a hospital.

Russia’s Health Ministry said about 50 people were injured, and Markin said 34 were hospitalized, many in grave condition.

The Interfax news agency said that the suspected bomber’s head was found at the site of explosion, which would allow security agencies to quickly identify her.

Female suicide bombers, many of whom were widows or sisters of rebels, have mounted numerous attacks in Russia. They often have been referred to as “black widows.”

In October, a female suicide bomber blew herself up on a city bus in Volgograd, killing six people and injuring about 30. Officials said that attacker came from the province of Dagestan, which has become the center of an Islamist insurgency that has spread across the region after two separatist wars in Chechnya. She took a Moscow-bound bus from Dagestan, but left it in Volgograd for an unclear reason and took a local bus, where she detonated her explosives.

As in Sunday’s blast, her bomb was rigged with shrapnel that caused severe injuries.

It wasn’t immediately clear where Sunday’s bomber came from, but officials in Dagestan were checking whether the attacker could come from the region, according to the ITAR-Tass news agency.

Chechnya has become more stable under the steely grip of its Moscow-backed strongman, who incorporated many of the former rebels into his feared security force. But Dagestan, the province between Chechnya and the Caspian Sea, has evolved as the epicenter of the rebellion, with near daily attacks on police and other officials.

The Kremlin replaced Dagestan’s provincial leader earlier this year, and the new leader abandoned his predecessor’s attempts at reconciliation and his efforts to persuade some of the rebels to surrender in exchange for amnesty.

Images caught by a security camera facing the station, broadcast by Rossiya 24 television, showed the moment of explosion: a bright orange flash inside the station behind the main gate followed by plumes of smoke.

“Pieces of flesh mixed with shards of glass and smoke billowed from inside, I didn’t even understand at first what was going on,” said local resident Svetlana Zabotko who witnessed the blast.

On Friday, three people were killed when a car rigged with explosives blew up on a street in Pyatigorsk, the center of a federal administrative district intended to stabilize the North Caucasus region.

Following Sunday’s explosion, the Interior Ministry ordered police to beef up patrols at railway stations and other transport facilities across Russia.

Russia in past years has seen a series of terror attacks on buses, trains and airplanes, some carried out by suicide bombers.

Twin bombings on the Moscow subway in March 2010 by female suicide bombers killed 40 people and wounded more than 120. In January 2011, a male suicide bomber struck Moscow’s Domodedovo Airport, killing 37 people and injuring more than 180.

Umarov, who had claimed responsibility for the 2010 and 2011 bombings, ordered a halt to attacks on civilian targets during the mass street protests against President Vladimir Putin in the winter of 2011-12. He reversed that order in July, urging his men to “do their utmost to derail” the Sochi Olympics which he described as “satanic dances on the bones of our ancestors.”

The group that called itself Anonymous Caucasus said in a statement posted Friday on the Caucasus rebel web site, kavkazcenter.com, that it will launch cyber-attacks to avenge Russia’s refusal to acknowledge the 19th century expulsion of Chirkassians, one of the ethnic groups in the Caucasus.

Russian authorities have introduced some of the most extensive identity checks and sweeping security measures ever seen at an international sports event.

Anyone wanting to attend the games that open on Feb. 7 will have to buy a ticket online from the organizers and obtain a “spectator pass” for access. Doing so will require providing passport details and contacts that will allow the authorities to screen all visitors and check their identities upon arrival.

The security zone created around Sochi stretches approximately 100 kilometers (60 miles) along the Black Sea coast and up to 40 kilometers inland. Russian forces include special troops equipped for patrolling the forested mountains towering over the resort, drones to keep constant watch over Olympic facilities and speed boats to patrol the coast.

The security regime includes a ban on the entry of all cars from outside the zone starting one month before the games and ending only one month after they end.

WASHINGTON – Hillary Clinton challenged Congress on Thursday to combat fake and misleading news on social media, using a post-election appearance to tackle an issue that gripped her presidential campaign and culminated with a shooting incident Sunday in Northwest Washington.